Synthesis of δ-MnO₂ via ozonation routine for low temperature formaldehyde removal

Nowadays, it is still a challenge to prepared high efficiency and low cost formaldehyde (HCHO) removal catalysts in order to tackle the long-living indoor air pollution. Herein, δ-MnO₂ is successfully synthesized by a facile ozonation strategy, where Mn²⁺ is oxidized by ozone (O₃) bubble in an alkaline solution. It presents one of the best catalytic properties with a low 100% conversion temperature of 85°C for 50 ppm of HCHO under a GHSV of 48,000 mL/(g·hr). As a comparison, more than 6 times far longer oxidation time is needed if O₃ is replaced by O₂. Characterizations show that ozonation process generates a different intermediate of tetragonal β-HMnO₂, which would favor the quick transformation into the final product δ-MnO₂, as compared with the relatively more thermodynamically stable monoclinic γ-HMnO₂ in the O₂ process. Finally, HCHO is found to be decomposed into CO₂ via formate, dioxymethylene and carbonate species as identified by room temperature in-situ diffuse reflectance infrared fourier transform spectroscopy. All these results show great potency of this facile ozonation routine for the highly active δ-MnO₂ synthesis in order to remove the HCHO contamination.